A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
During operation of the lithographic apparatus there may be parts of the lithographic apparatus that are cooled and/or heated, for instance due to dissipation of heat in electromagnetic actuators or irradiation by an optical beam etc. Currently these parts are cooled by using e.g. liquid, such as water, which is pumped around in a circuit. The water extracts heat from the part to be cooled while passing the part after which the collected heat is dumped, i.e. removed, using a heat exchanger downstream of the part, such that the water is able to return to the part to extract heat again.
The cooling efficiency of this method depends on the flow type in the channels of the circuit at or near the part. A turbulent flow of the water has a higher heat transfer coefficient than a laminar flow of the water. Using small channels, which is beneficial from a design and weight point of view, has the disadvantage that a laminar flow is formed and thus decreases the cooling efficiency. The cooling efficiency of the method may be that poor that the temperature of the part raises to an undesired level which influences other parts, e.g. by heat radiation, and thereby reduces the obtainable accuracy of the lithographic apparatus.